1. Field of the Invention
The present invention relates to a solid-state imaging device and a production method thereof, more specifically, the present invention relates to a charge transfer electrode of a solid-state imaging device.
2. Background Art
The solid-state imaging device utilizing a charge coupled device (CCD) used for an area sensor and the like has a photoelectric conversion section comprising a photodiode or the like and a charge transfer section equipped with a charge transfer electrode for transferring a signal charge from the photoelectric conversion section. As for the charge transfer electrode, plural charge transfer electrodes are adjacently disposed on a charge transfer path formed on a semiconductor substrate and sequentially driven.
With the recent downsizing of a camera, demands for high resolution and high sensitivity of a solid-state imaging device are more and more increasing, and the number of imaging pixels has been increased to gigapixels or more.
In order to elevate the sensitivity, there is employed a method where a high refractive index film for antireflection is formed on a photodiode (photoelectric conversion section) and for the purpose of reducing smears, the region except for the top of the photoelectric conversion section is light-shielded by using a tungsten light-shielding film having an opening in the photoelectric conversion section.
For example, a method of forming an antireflection film to cover the light-receiving region of the photoelectric conversion section and after patterning, forming a light-shielding film thereon has been proposed.
According to this structure, a light-shielding film 7 can be formed to unfailingly cover the charge transfer section, but since a silicon oxide film is formed below the light-shielding film so as to ensure electrical withstand voltage with the charge transfer electrode, there is a problem that multiple reflection is generated between the light-shielding film and the silicon substrate, and the obliquely incident light run into the charge transfer path worsens the smear. The smear is a phenomenon such that when strong light is irradiated on a solid-state imaging device, the light reaches the charge transfer section and due to an electric charge generated therein, a band-like imaging defect appears on the screen. Therefore, in increasing the number of pixels, it becomes an important problem to cover the charge transfer section with a light-shielding film and thoroughly open the opening of the photoelectric conversion section.
To solve this problem, a method of forming a light-shielding film below the antireflection film has been proposed. However, also in this case, in order to ensure electrical withstand voltage between the tungsten light-shielding film and the charge transfer electrode, an insulating film such as silicon oxide film needs to be formed below the light-shielding film.
Consequently, a method of forming a pattern of an antireflection film to cover the top of the photoelectric conversion section, and forming a light-shielding film to self-align with the edge thereof has been proposed in JP-A-2004-140309 (the term “JP-A” as used herein means an “unexamined published Japanese patent application).
In this method, as seen in FIGS. 8A to 8E showing the production process thereof, a silicon oxide film as a flattening film 10 is formed on an antireflection film 5 and patterned by lithography to selectively remain only on the photodiode region, and a light-shielding film 9 is then formed.
According to this structure, a light-shielding film 9 can be formed to unfailingly cover the charge transfer section, thereby eliminating the effect of smears, and a high-sensitivity low-smear solid-state imaging device can be provided.
However, this method requires to take an alignment margin of the photolithography, and this stands as an obstacle hampering microfabrication.
As shown in FIG. 8C, the flattening film formed on the antireflection film needs to be patterned by photolithography. In this photolithography process, overlap accuracy is required and if inaccurate, the sensitivity is greatly reduced.
That is, there is a problem that despite the need for extremely high-level overlap accuracy, enhancement of the overlap accuracy by photolithography is very difficult.
In this way, in the conventional solid-state imaging device shown in FIGS. 8A and 8E, displacement at the patterning of an antireflection film and a flattening film is a serious problem and sometimes causes fluctuation of sensitivity or worsening of smears.